Pressurized water nuclear reactors include a vessel containing the core of the reactor which is submerged in the pressurized water for cooling the reactor.
The generally cylindrical vessel of the reactor has a domed bottom head of generally hemispherical shape and a head which is also of hemispherical shape for closing the upper part of the vessel.
The domed bottom head of the vessel is pierced with openings which are parallel to the axis of the vessel, and in each of which an axially directed tubular part for passing through the vessel bottom head is fixed by welding.
These tubular through parts or penetrations of the vessel bottom head have an end projecting below the domed bottom head of the vessel, which is connected to a measuring pipe allowing the vessel bottom head to be joined to an instrumentation room arranged in the structure of the building of the reactor. The vessel bottom head penetrations also have a part projecting above the domed bottom head, inside the vessel, which is intended to engage, with clearance, inside a channel for guiding the lower internals of the reactor which are mounted inside the vessel and which hold the core assemblies.
Each of the guide assemblies, consisting of a measuring pipe, a vessel bottom head penetration, of tubular shape, and the corresponding channel for guiding the lower internals, provides passage for a thimble into which a probe can be introduced, making it possible to carry out measurements inside the core, such as neutron flux measurements or temperature measurements, during operation of the reactor.
In order to increase the reliability and safety operation of nuclear reactors and in order to extend the lifetime of these reactors, the operators need to carry out an increasing number of examinations of the various elements constituting the nuclear reactor. When damage or defects such as cracks are detected on an element of the nuclear reactor, the defective element may possibly be repaired, deactivated or replaced, depending on the seriousness of the defect.
In particular, the condition of the through penetrations of the vessel bottom head is periodically examined, in order to check their integrity after a certain running time of the reactor, especially in the region where the vessel bottom head penetrations are welded onto the vessel bottom head.
The vessel bottom head penetrations are generally made of a nickel alloy, so that, in order to fasten them by welding on the vessel bottom head, it is necessary to deposit a layer of nickel alloy in the bottom of a bevel around the through opening of the vessel bottom head, in order to link the welding metal with the vessel bottom head. The welding metal consisting of a nickel alloy is deposited in the bevel, around the vessel bottom head penetration.
Depositing the welding metal in contact with the vessel bottom head penetration produces heating of this penetration, which heating may be relatively significant.
When the vessel bottom head penetration is made in certain grades of nickel alloy, there may be a risk of the vessel bottom head penetrations cracking in the welding region, after a certain time of use of the nuclear reactor.
In the event that a defect is detected on the internal surface, on the external surface or on the weld of a vessel bottom head penetration, the seriousness and the depth of this defect is evaluated and the defect may possibly be repaired, for example by boring the metal of the penetration to a certain depth in the cracked region, which may be followed by protection of the excavated region by depositing a repair metal or fitting a sleeve.
In certain cases, it is preferable to replace the vessel bottom head penetration completely rather than repairing it.
In the event of such a replacement, it is possible to guarantee perfect integrity of the replacement part and, to replace the defective nickel alloy part, which is susceptible to cracking, with a part made of improved alloy which exhibits little or no susceptibility to cracking corrosion, inside the nuclear reactor.
Various methods have been proposed and envisaged for replacing a vessel bottom head penetration of a nuclear reactor, consisting in disconnecting the weld between the defective penetration and the vessel bottom head, by machining, in extracting the defective vessel bottom head penetration from the through opening of the vessel bottom head and in fixing a vessel bottom head penetration into the through opening by welding.
All the prior art methods have draw-backs, because of the difficulty of implementing the operations of welding the replacement part, and of inspecting the welds carried out and the interactions between the weld and the replacement part, during welding and during cooling of the welding metal.